feat(tex): section-aware multi-file .tex ingest (R-C)

Flatten multi-file arXiv LaTeX (\input/\include) in arxiv.fetch_source so the full body is assembled rather than just the primary file's include skeleton, and add section-aware chunking (tex.chunk_sections) so .tex chunks never span a \section boundary and are labelled by their real heading. The ingest .tex path now produces (section, chunk) pairs, quality-gated together so labels stay aligned; the stored 'section' column gains genuine signal instead of mostly 'body' (serves DQ-3 fidelity + audit R-12).

Adds scripts/rc_tex_reingest.py to re-ingest arXiv papers from .tex (dry-run by default; replaces chunks). Tests: flatten_inputs, chunk_sections, multi-file fetch_source, section-label ingest, is_arxiv_id. Full suite 365 passed; ruff + mypy clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/sources/test_arxiv.py

@@ -53,6 +53,39 @@ def test_fetch_source_returns_latex(monkeypatch: pytest.MonkeyPatch) -> None:
     assert "Hello world" in result
 
 
+def test_fetch_source_flattens_multifile(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Multi-file projects: \\input/\\include directives are inlined so the body
+    from the included files is returned, not just the primary skeleton (R-C)."""
+    main = (
+        b"\\documentclass{article}\\begin{document}"
+        b"\\input{sections/intro}\\include{proof}\\end{document}"
+    )
+    tar_bytes = _make_tar_gz(
+        {
+            "main.tex": main,
+            "sections/intro.tex": b"INTRODUCTION BODY TEXT",
+            "proof.tex": b"PROOF BODY TEXT",
+        }
+    )
+
+    def mock_get(
+        url: str,
+        *,
+        timeout: int = 60,
+        follow_redirects: bool = True,
+    ) -> httpx.Response:
+        return httpx.Response(200, content=tar_bytes)
+
+    monkeypatch.setattr(httpx, "get", mock_get)
+
+    result = arxiv.fetch_source("2301.07041")
+
+    assert result is not None
+    assert "INTRODUCTION BODY TEXT" in result  # \input was inlined
+    assert "PROOF BODY TEXT" in result  # \include was inlined
+    assert "\\input" not in result and "\\include" not in result
+
+
 # ---------------------------------------------------------------------------
 # fetch_source — 404 → None
 # ---------------------------------------------------------------------------